Reproductive efficiency, particularly as it relates to litter size, is the major limiting factor in the efficient production of pork as well as most other livestock animals. Genetic variability exists for several reproductive measures. Average litter size among breeds pigs varies from 4-16 pigs per litter. Mean age at puberty varies from 3 to 7 months of age. This genetic variability within breeds suggests that genetic improvement in reproduction is possible. The number of pigs born alive in the United States averages approximately 9.5 pigs per litter. Heritability for litter size is low (10%-15%), and standard genetic methods of selecting breeding females on the basis of past litter size have not been effective. Therefore, there is a need for an approach that deals with selection for reproductive traits at the cellular or DNA level.
Chinese breeds are known for reaching puberty at an early age and for their large litter size. American breeds are known for their greater growth rates and leanness. Thus, it would be desirable to combine the best characteristics of both types of breeds, thereby improving the efficiency of U.S. pork production. These efforts would be greatly assisted by the discovery of genes or genetic markers that are associated with improved reproductive traits such as increased litter size in pigs.
RFLP analysis has been used by several groups to study pig DNA. Jung et al., Theor. Appl. Genet., 77:271-274 (1989), incorporated herein by reference, discloses the use of RFLP techniques to show genetic variability between two pig breeds. Polymorphism was demonstrated for swine leukocyte antigen (SLA) Class I genes in these breeds. Hoganson et al., Abstract for Annual Meeting of Midwestern Section of the American Society of Animal Science, Mar. 26-28, 1990, incorporated herein by reference, reports on the polymorphism of swine major histocompatibility complex (MHC) genes for Chinese pigs, also demonstrated by RFLP analysis. Jung et al. Animal Genetics, 26:79-91 (1989), incorporated herein by reference, reports on RFLP analysis of SLA Class I genes in certain boars. The authors state that the results suggest that there may be an association between swine SLA/MHC Class I genes and production and performance traits. They further state that the use of SLA Class I restriction fragments, as genetic markers, may have potential in the future for improving pig growth performance.
Further, U.S. Pat. No. 5,550,024 to Rothschild et al. discloses a polymorphism in the pig estrogen receptor gene which is associated with larger litter size, the disclosure of which is incorporated herein by reference.
Another pig hormone related to beneficial reproductive traits is Prolactin. Prolactin (PRL) is an anterior pituitary peptide hormone involved in many different endocrine activities, but is essential for reproductive success. Use of polymorphic loci in the prolactin receptor gene as markers for increased litter size is described and disclosed in U.S. patent application Ser. No. 08/812,208, the disclosure of which is hereby incorporated by reference.
The present invention provides genetic markers, based upon the discovery of polymorphisms in reproductive genes, which relate to improved reproductive traits such as testicular size, sperm volume, concentration of sperm, sperm quality, libido and breeding aggressiveness for males. For females reproductive traits include litter size, number born alive, litter birth weight, number weaned, age at puberty, weaning to oestrus, farrowing interval, ovulation rate, uterine capacity and embryo survival in pigs. This will permit genetic typing of pigs for their reproductive genes and for determination of the relationship of specific genes and markers to reproductive traits. It will also permit the identification of individual males and females that carry beneficial genotypes. In the case of females it would permit that a female would be expected to produce a litter size larger than the average earlier than average or healthier than average for their breed, or in the case of males, for their female offspring to have the beneficial traits. Thus, the markers will be selection tools in breeding programs to develop lines and breeds that produce litters with favorable reproductive phenotypes.
It is an object of the invention to provide a method of screening pigs to determine those more likely to produce larger litters.
Another object of the invention is to provide a method for identifying genetic markers for reproductive traits such as pig litter size.
A further object of the invention is to provide genetic markers for pig litter size.
Yet another object of the invention is to provide a kit for evaluating a sample of pig DNA for specific genetic markers associated with favorable reproductive traits.
Additional objects and advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The objects and advantages of the invention will be attained by means of the instrumentality's and combinations particularly pointed out in the appended claims.